arXiv subjects

Will Barker

Publications and source records attributed to Will Barker.

25 records · Page 2Linked to original sources

arXiv2024

Inflationary and Gravitational Wave Signatures of Small Primordial Black Holes as Dark Matter

Mounting evidence suggests that the semi-classical description of a black hole breaks down at the latest after losing an O(1) fraction of its mass. As a result, effects such as memory burden can slow down evaporation so that small primordial black holes (PBHs), in particular those in the mass range 10^6 g to 10^9 g, become viable dark matter candidates. In this paper, we investigate the production of PBHs from a prototype model of polynomial inflation with a non-minimal coupling to gravity. We show that a sufficiently small PBH mass alleviates any tension with CMB observations. Moreover, we develop efficient numerical procedures to identify model parameters and evolve Mukhanov-Sasaki modes to place bounds on the scalar-induced stochastic gravitational wave (GW) background. Whilst we identify some prospects for observation with future GW detectors, our results highlight the need to develop new experiments for high-frequency GW detection in the ~kHz to ~MHz range. Finally, we demonstrate that previously-used ansätze for modelling the power spectrum only yield a reliable approximation for the GW signal if some input from inflation is used.

astro-ph.CO↗

arXiv2024

Buchdahl bound, photon ring, ISCO and radial acceleration in Einstein-æther theory

Spherically symmetric Einstein-æther (EÆ) theory with a Maxwell-like kinetic term is revisited. We consider a general choice of the metric and the æther field, finding that:~(i) there is a gauge freedom allowing one always to use a diagonal metric; and~(ii) the nature of the Maxwell equation forces the æther field to be time-like in the coordinate basis. We derive the vacuum solution and confirm that the innermost stable circular orbit (ISCO) and photon ring are enlarged relative to general relativity (GR). Buchdahl's theorem in EÆ theory is derived. For a uniform physical density, we find that the upper bound on compactness is always lower than in GR. Additionally, we observe that the Newtonian and EÆ radial acceleration relations run parallel in the low pressure limit. Our analysis of EÆ theory may offer novel insights into its interesting phenomenological generalization: Æther--scalar--tensor theory (ÆST).

gr-qc↗

arXiv2024

PSALTer: Particle Spectrum for Any Tensor Lagrangian

We present the PSALTer software for efficiently computing the mass and energy of the particle spectrum for any (e.g. higher-rank) tensor field theory in the Wolfram Language. The user must provide a Lagrangian density which is expanded quadratically in the fields around a Minkowski vacuum, is linear in the coupling coefficients, and otherwise built from the partial derivative and Minkowski metric. PSALTer automatically computes the spin-projection operators, saturated propagator, bare masses, residues of massive and massless poles and overall unitarity conditions in terms of the coupling coefficients. The constraints on the source currents and total number of gauge symmetries are produced as a by-product. We provide examples from scalar, vector, tensor and gauge theories of gravity. Each example, including spectra of higher-spin modified gravity theories, may be obtained on a personal computer in a matter of minutes. The software is also parallelised for use on high-performance computing resources. The initial release allows for parity-preserving operators constructed from fields of up to rank three: this functionality will be extended in future versions. PSALTer is a contribution to the xAct project.

hep-th↗

arXiv2024

Consistent particle physics in metric-affine gravity from extended projective symmetry

It is well-known that the Einstein-Hilbert action exhibits a projective invariance in metric-affine gravity, generated by a single vector (just like diffeomorphisms). However, this symmetry offers no protection against formulating inconsistent models, e.g., with ghost and strong coupling problems. In this letter, we observe that non-minimal kinetic terms of Dirac spinors point to a new extended projective (EP) symmetry generated by a pair of vectors. We prove that the most general EP-invariant theory (at most quadratic in field strengths) is naturally free from all pathologies. Its spectrum only features the massless graviton and a single additional scalar field arising from the square of the Holst curvature. The scalar potential is suitable for inflation and our model moreover contains effective 4-Fermi interactions capable of producing fermionic dark matter. Finally, we point out an alternative double-vector symmetry that similarly leads to a healthy theory with a propagating vector field.

hep-th↗

arXiv2024

Einstein-Proca theory from the Einstein-Cartan formulation

We construct a theory of gravity in which a propagating massive vector field arises from a quadratic curvature invariant. The Einstein-Cartan formulation and a partial suppression of torsion ensure the absence of ghost and strong-coupling problems, as we prove with nonlinear Lagrangian and Hamiltonian analysis. Augmenting General Relativity with a propagating torsion vector, our theory provides a purely gravitational origin of Einstein-Proca models and constrains their parameter space. As an outlook to phenomenology, we discuss the gravitational production of fermionic dark matter.

hep-th↗

arXiv2023

Particle spectra of gravity based on internal symmetry of quantum fields

We examine the weak-field, zero-coupling limit of Yang--Mills gravity as recently formulated by Partanen and Tulkki, viewed as a free quantum field theory. In this approximation the theory has a ghostly teleparallel vacuum. We suggest that bimetric, vacuum expectation value, or finite-coupling extensions should be investigated.

hep-th↗

arXiv2023

Manifestly covariant variational principle for gauge theories of gravity

A variational principle for gauge theories of gravity is presented, which maintains manifest covariance under the symmetries to which the action is invariant, throughout the calculation of the equations of motion and conservation laws. This is performed by deriving explicit manifestly covariant expressions for the Euler--Lagrange variational derivatives and Noether's theorems for a generic action of the form typically assumed in gauge theories of gravity. The approach is illustrated by application to two scale-invariant gravitational gauge theories, namely Weyl gauge theory (WGT) and the recently proposed `extended' Weyl gauge theory (eWGT), where the latter may be considered as a novel gauging of the conformal group, but the method can be straightforwardly applied to other theories with smaller or larger symmetry groups. The approach also enables one easily to establish the relationship between manifestly covariant forms of variational derivatives obtained when one or more of the gauge field strengths is set to zero either before or after the variation is performed. This is illustrated explicitly for both WGT and eWGT in the case where the translational gauge field strength (or torsion) is set to zero before and after performing the variation, respectively.

gr-qc↗